The aim of this study is to compare two methods of measuring the change in calf venous volume during head-up tilt. We hypothesize that 1) head up tilt will be associated with an increase in the circumference of the calf (venous pooling), 2) the change in circumference of the calf during head-up tilt will be correlated with the change in number of counts obtained from the calf during head up tilt in patients with a radionuclide labeled blood pool.